1. Field of Invention
This invention relates to a method for determining the start commutation in rotary and linear synchronous and reluctance servo-drives with the aid of two coupled control loops or circuits.
2. Description of the Prior Art
Direct current machines have a collector and two or more brushes serving as commutators. In the direct current machine, the said commutator ensures that the current always flows through the windings in the armature in such a way that the magnetic fields resulting from the current attract so that a torque is generated, and in this way the motor shaft rotates. In the case of synchronous servo-drives, which nowadays have largely replaced direct current machines due to certain decisive advantages (approximately 50% smaller construction volume for the same power with no need of maintaining the brushes), said commutator is missing. Said commutator must be simulated in the servo amplifier, complex electronics also providing the current control, speed control and position control of the synchronous servo-drive, is nowadays almost always implemented with the aid of Clarke-Park transformation. For simulating the commutator, switching on the electronics of the servo amplifier initially requires the rotor position relative to the stator, hereinafter called “start commutation”. This information can be provided with corresponding costs by a measuring system or use is made of the method according to DE 196 04 701 C1, which requires additional hardware.
EP 784 378 A2 discloses a method for initializing the start commutation for a synchronous motor with incremental position sensors in which a position controller and a current controller are used, and with said position controller the motor movement is minimized on the basis of the commutation initialization. Thus, a quantity consisting of moment-forming and flow-forming components are impressed in the direction of the estimated rotor position angle in that the moment-forming component is selected as zero, and the flow-forming component is chosen with a value higher than zero (estimated). If the rotor position angle estimated value diverges from the actual rotor position angle, a torque is generated which causes a rotor movement in the direction of the estimated rotor position. Thus, in the least favourable estimated case, an angular movement of the rotor is required which does not make it possible to start it from the stationary, i.e. fixed braked state.
It is disadvantageous in the prior art that implementation takes place in the form of additional hardware which, as a function of the data of the electric drive, must be hardware-adapted, and that the electrical machine must twice perform an uncontrolled movement in the millimetre range so that such methods cannot function when the brake is applied. It is also disadvantageous that the position change resulting from the uncontrolled movement is maintained after finding the start commutation, and the implemented mathematical equations necessary in the servo amplifier for vector control using Clarke-Park transformation cannot be used for finding the start commutation which makes the additional hardware costs considerable.
The problem of the invention is to provide a simple method making it possible in the case of dynamic movement freedom and zero movement freedom in the stationary end value, using the hardware and software which is always present in any vector-controlled electric drive, to determine the start commutation, and after producing the latter to keep the electric drive by means of electromotive force in the position which it occupied prior to the start of finding the start commutation.